The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling a continuously variable transmission having a torque converter with a lockup clutch.
A continuously variable transmission having a torque converter with a lockup clutch is known. The torque converter operates to multiply the torque of an engine, so that the vehicle can be smoothly started. After the start of the vehicle, the torque converter is preferably locked up thereby preventing a loss of power and improving the fuel consumption. Thus, various control systems for controlling the lockup clutch have been proposed in order to effectively use such characteristics.
Japanese Patent Laid Open No. 55-109854 discloses a control system for a lockup clutch wherein the lockup clutch is engaged depending on the speed ratio of the speed of an input member to the speed of an output member of the torque converter to overcome the inefficiency of the torque converter.
However, if such a control system, in which the lockup clutch is operated regardless of the control of the transmission ratio, is applied to the continuously variable transmission, various problems arise.
For example, a relationship between the engine torque characteristic and the rotational speed ratio characteristic of the torque converter is shown in FIG. 6a. The torque converter characteristic is shown in FIG. 6b. The rotational speed ratio e is the ratio of speed of an input member of the torque converter to speed of an output member thereof, which is obtained from the ratio of engine speed Ne to drive pulley speed Np (e=Ne/Np). The converter operational zone and the coupling zone is divided at a point where the rotational speed ratio e is 0.85. The converter operational zone is in the range of 0.ltoreq.e&lt;0.85. The coupling zone is in the range of 0.85.ltoreq.e.
As shown in FIG. 6a, if the transmission ratio is fixed at a point A while the vehicle is driven, the drive pulley speed Np increases with increase of the vehicle speed. Accordingly, the torque converter condition enters into the coupling zone as shown by an arrow A'. The torque converter is preferable to be locked up in the coupling zone. On the other hand, if the transmission ratio is upshifted under this driving condition, the drive pulley speed Np decreases. Thus, the torque converter enters into the operational zone as shown by an arrow A". As a result, the lockup of the torque converter is not effected.
Accordingly, it is preferable for the lockup control to cooperate with the transmission control. It is important to fix the transmission ratio so that the torque converter enters into the coupling zone, in order to lock up the torque converter.
When the transmission starts to change at starting of the vehicle, acceleration of the vehicle is multiplied by the upshifting of the transmission, causing a shock by +G as shown in FIG. 7. On the other hand, when the torque converter is locked up, the vehicle speed reduces, causing a shock by -G. It is desired to prevent these shocks.